1. Field of the Invention
The present invention is related to a sheet formation system for a fourdrinier machine for producing, e.g., paper or cardboard, and to a method for reducing edge waves in a sheet formation system.
2. Discussion of Background Information
Sheet formation systems for fourdrinier machines include an endless screen (sieve) for formatting paper or cardboard webs that is guided or runs over an open, flat table surface. Pulp heavily diluted with water, i.e., pulp suspension, is added to a headbox that produces a pulp suspension jet through a nozzle that extends along the width of the machine and that directs the pulp suspension jet onto the endless screen. Subsequently, fluid is drained through the screen and a pulp web is produced. The pulp web is then guided through the machine and is processed into a finished paper web or cardboard web.
After an impact point of the pulp suspension jet upon the screen, the pulp suspension at the edges of the screen tends to run off laterally. For this reason, in the area of the edges of the screen, i.e., after the nozzle of the headbox, so called deckle plates (or a deckle pan assembly) are attached to prevent both a lateral flow of the pulp suspension and a thinning of the produced paper or cardboard web in this area.
Deckle plates of the type generally discussed above are known from, e.g., Patent Application DE 43 34 641 A1, the disclosure of which is expressly incorporated by reference herein in its entirety. This document discloses as its object preventing permanent accumulation of fiber layers that form in the area of the edges of a flowing pulp suspension. The accumulation of fiber layers results from drying and these dried layers may flake into the fresh pulp suspension. For this purpose, the noted document discloses providing spray nozzles in the area of the deckle plate to cover the deckle plate surfaces with a flowing aqueous film, so as to prevent drying-on of pulp accumulations. The deckle plate itself is comprised of a flat strip or deckle edge which is strengthened in its directional stability by hollow sections aligned in a machine direction.
A major problem with the known deckle plate is that, in the area of the edge, secondary flows develop from the pulp suspension flowing past the boundary layers, and these secondary flows disadvantageously produce wavy edges in the pulp suspension that run toward the machine center.